A number of immunotherapeutic agents have been described for use in cancer treatment, including therapeutic monoclonal antibodies (mAbs), immunoconjugated mAbs, radioconjugated mAbs and bi-specific T-cell engagers.
Typically these immunotherapeutic agents target a single antigen: for instance, Rituximab targets CD20; Myelotarg targets CD33; and Alemtuzumab targets CD52.
However, it is relatively rare for the presence (or absence) of a single antigen effectively to describe a cancer, which can lead to a lack of specificity. Targeting antigen expression on normal cells leads to on-target, off-tumour toxicity.
Most cancers cannot be differentiated from normal tissues on the basis of a single antigen. Hence, considerable “on-target off-tumour” toxicity occurs whereby normal tissues are damaged by the therapy. For instance, whilst targeting CD20 to treat B-cell lymphomas with Rituximab, the entire normal B-cell compartment is depleted, whilst targeting CD52 to treat chronic lymphocytic leukaemia, the entire lymphoid compartment is depleted, whilst targeting CD33 to treat acute myeloid leukaemia, the entire myeloid compartment is damaged etc.
The predicted problem of “on-target off-tumour” toxicity has been bourne out by clinical trials. For example, an approach targeting ERBB2 caused death to a patient with colon cancer metastatic to the lungs and liver. ERBB2 is over-expressed in colon cancer in some patients, but it is also expressed on several normal tissues, including heart and normal vasculature.
In some cancers, a tumour is best defined by presence of one antigen (typically a tissue-specific antigen) and the absence of another antigen which is present on normal cells. For example, acute myeloid leukaemia (AML) cells express CD33. Normal stem cells express CD33 but also express CD34, while AML cells are typically CD34 negative. Targeting CD33 alone to treat AML is associated with significant toxicity as it depletes normal stem cells. However, specifically targeting cells which are CD33 positive but not CD34 positive would avoid this considerable off-target toxicity.
There is thus a need for immunotherapeutic agents which are capable of more targeting to reflect the complex pattern of marker expression that is associated with many cancers.
Chimeric Antigen Receptors (CARs)
Chimeric antigen receptors are proteins which graft the specificity of a monoclonal antibody (mAb) to the effector function of a T-cell. Their usual form is that of a type I transmembrane domain protein with an antigen recognizing amino terminus, a spacer, a transmembrane domain all connected to a compound endodomain which transmits T-cell survival and activation signals (see FIG. 1A).
The most common form of these molecules are fusions of single-chain variable fragments (scFv) derived from monoclonal antibodies which recognize a target antigen, fused via a spacer and a trans-membrane domain to a signaling endodomain. Such molecules result in activation of the T-cell in response to recognition by the scFv of its target. When T cells express such a CAR, they recognize and kill target cells that express the target antigen. Several CARs have been developed against tumour associated antigens, and adoptive transfer approaches using such CAR-expressing T cells are currently in clinical trial for the treatment of various cancers.
However, the use of CAR-expressing T cells is also associated with on-target, off tumour toxicity. For example, a CAR-based approach targeting carboxy anyhydrase-IX (CAIX) to treat renal cell carcinoma resulted in liver toxicity which is thought to be caused by the specific attack on bile duct epithelial cells (Lamers et al (2013) Mol. Ther. 21:904-912.
There is thus a need for alternative CAR-based T cell approaches with greater selectivity and reduced on target, off tumour toxicity.